This invention relates to a combining circuit for use in a diversity receiving system.
A combining circuit combines first and second received signals from two independent antennas to produce a combined signal. Various combining circuits of the type are already known. By way of example, a combining circuit is disclosed in Japanese Unexamined Patent Prepublication No. 27752/80, by Nobuo Katayamao. In the combining circuit disclosed by Katayama, first and second output signals given from two independent receivers are applied to first and second variable gain amplifiers. The first and the second variable gain amplifiers produce first and second amplified outputs signals which are added together at an adder. The adder produces an added output signal which is drawn out from an output terminal via an automatic gain control circuit. A serial circuit consisting of a first amplifier, an inversional circuit, and a second amplifier is installed between the first and the second variable gain amplifiers. First and second control voltages sent from the two independent receivers are applied to a subtractor circuit for subtraction. The subtractor circuit has an output which is connected to the joint of the first amplifier and the inversional circuit forming the serial circuit. As a result, the linear relation is available for the gain to the gain control voltage of the first and the second variable gain amplifiers, thus obtaining the optimum compound ratio over a wide range with no special property required.
Another combining circuit is disclosed in Japanese Unexamined Patent Prepublication No. 157139/81, by Atsushi Muromoto. In the combining circuit disclosed by Muromoto, first and second receivers demodulate first and second received signals from two antennas into first and second baseband signals, respectively. Connected to the first and the second receivers, first and second control voltage generators generate first and second control voltages according to the signal-to-noise ratio of the first and the second received signals, respectively. The first and the second control voltages are supplied to an auxiliary circuit which comprises a maximum control voltage generator, a reference voltage comparator, and first and second differential amplifiers. The maximum control voltage generator extracts the maximum control voltage from the first and second control voltages and provides it to the reference voltage comparator. The reference voltage comparator produces a difference voltage between the maximum control voltage and a reference voltage. The difference voltage is applied to an input of each of the first and the second differential amplifiers that amplify each diversity route. The first and the second control voltages are applied to another inputs of the first and the second differential amplifiers, respectively. The first differential amplifier produces a first differential voltage between the first control voltage and the difference voltage while the second differential amplifier produces a second differential voltage between the second control voltage and the difference voltage. The first and the second baseband signals are supplied to first and second specific square attenuators which are controlled by the first and the second differential voltages, respectively.
Still another combining circuit is disclosed in an article which is contributed by Tomohiro Dohi et al to IMSC (1993), pages 455-460, and which has a title of "Performance of the Unique-Word-Reverse-Modulation Type Demodulator for Mobile Satellite Communications." The combining circuit disclosed by Dohi et al, comprises two RF/IF circuits and a pre-detection signal combiner. Signal sequences are received by two antennas and converted into baseband analog signal sequences by quadrature detection in the two RF/IF circuits each of which consists of a frequency converter, a band pass filter (BPF), and an automatic gain control (AGC) amplifier. In the signal combiner, each analog signal sequence is converted into a 12 bit digital signal sequence with identical timing. Each branch's signal sequence is combined to improved the received bit energy-to-noise power density ratio (E.sub.b /N.sub.0).
At any rate, as will later be described in conjunction with FIG. 1, a conventional combining circuit comprises first and second frequency converting sections, first and second main amplifying sections, and a signal combining section. The first and the second frequency converting sections are supplied with first and second received signals from two independent antennas, respectively. The first and the second frequency converting sections frequency convert the first and the second received signals into first and second frequency-converted signals having first and second output levels, respectively. Connected to the first and the second frequency converting sections, respectively, the first and the second main amplifying sections mainly amplify the first and the second frequency-converted signals by first and second amplification degrees in proportion to the first and the second output levels, respectively. The first and the second main amplifying sections produce first and second main amplified signals having first and second phases, respectively. Connected to the first and the second main amplifying sections, the signal combining section combines the first main amplified signal with the second main amplified signal with the first phase shifted into the second shift. The signal combining section produces a combined signal. The combined signal is supplied to a demodulator.
In general, the first and the second frequency converting sections have first and second converting gains which are different from each other. Under the circumstances, the first and the second frequency-converted signals have first and second noise power levels, respectively, which are different from each other. As a result, the conventional combining circuit is disadvantageous in that it is impossible to obtain the first and the second amplification degrees for the first and the second main amplifying sections having an optimal combining ratio therebetween.